1. Technical Field
The present invention relates to an organic transistor, a method for manufacturing the organic transistor, and an electronic apparatus including the organic transistor.
2. Related Art
As an alternative to thin-film field-effect transistors made of an inorganic material, such as silicon, organic thin-film field-effect transistors (hereinafter referred to as organic transistors) made of an organic semiconductor have received attention because of the following advantages:
(1) Unlike the inorganic semiconductor, organic semiconductors can be made in an extremely low-temperature process. Accordingly, organic semiconductors allow the use of flexible substrates, such as plastic substrates and film substrates, and thus, can provide light-weight flexible, non-breakable devices.
(2) The use of organic semiconductors allow a simple, short-time process for manufacturing devices, such as applying a liquid or printing, accordingly greatly reducing the manufacturing cost and equipment cost.
(3) Organic semiconductors can be selected from a wide variation of materials, and the material properties and device characteristics can be easily and radically changed by modifying the molecule structure of the material. In addition, a combination of different functions of the material can provide new functions and devices that cannot be provided by use of inorganic semiconductors.
Examples of the related art include: Japanese Unexamined Patent Application Publication Nos. 2004-47566, 2006-187706, and 2004-319982.
FIG. 5 shows a sectional view of a typical organic transistor. The organic transistor includes, on a substrate 10, a source electrode 11, a drain electrode 12, a semiconductor layer 13, a gate insulating layer 14, and a gate electrode 15. Arrow lines c1, c2, and c3 in the figure show the path through which carriers are conducted when the organic transistor is in an On state. In order to manufacture the organic transistor, the source electrode 11 and the drain electrode 12 are formed on the substrate 10, and then the semiconductor layer 13 is formed to an uniform thickness by, for example, spin coating. In addition, after coating the semiconductor layer 13 with the gate insulating layer 14, a gate electrode 15 is formed.
However, in such an organic transistor, the thickness of the semiconductor layer 13, which significantly influences the electrical characteristics, is difficult to control appropriately. It is accordingly difficult to obtain satisfying electric al characteristics. For example, the channel region (in which carriers are inducted, region corresponding to path c3) of an organic transistor is a small region having a thickness of about 1 to 5 nm in contact with the gate insulating layer 14. Accordingly, carriers first travel from the source electrode 11 to the channel region across an intrinsic semiconductor portion (path c1) having a high resistance, then through the channel region (path c3), and finally across another intrinsic semiconductor portion (path c2). If the semiconductor layer 13 has a large thickness, carriers have to travel through long paths of the intrinsic semiconductor portions (paths c1 and c2). Thus, the on-resistance of the organic transistor is increased.
On the other hand, an experiment has shown that it is preferable that the channel region have a large thickness. It is, in general, preferable that the channel region of a transistor made of an inorganic semiconductor have a small thickness because a large thickness of the channel region increases the number of traps in the layer. Unlike the inorganic semiconductor, it has been found that, in organic semiconductors, the channel region provides superior characteristics as the thickness is increased. Although the reason why a larger thickness leads to the superior effect is not clear, it may be because the interface between the semiconductor layer 13 and the gate insulating layer 14 is not easily affected by the underlying roughness, or a solid charge or trap level produced in the interface between the semiconductor layer 13 and its underlying layer (substrate 10 in FIG. 5)